This invention involves preparing composite parts by using an electron beam to selectively cure an electron-beam-curable thermoset resin. The invention advances the art in two related technical areas: fabrication of organic-matrix composites and rapid prototyping.
Fabrication of Advanced Organic-Matrix Composites
High-performance thermoset-matrix composite parts are used extensively in military aircraft, commercial aircraft, space vehicles, and sporting goods; and their use is expanding as manufacturing techniques improve and costs are reduced. These materials contain a high volume fraction (greater than about 50%) of continuous fiber reinforcement, which helps to give the materials the required strength and stiffness. There are a variety of methods for fabricating shaped parts from continuous fiber-reinforced plastic parts, including prepreg layup, resin transfer molding, pultrusion and compression molding, but all require the use of a tool (a mold, die, mandrel, etc.) to give the part its shape. (The term “pultrusion” comes from a combination of the words “pulling” and “extrusion.” Fibers and resin are pulled through a die of a desired shape. This is used to make long parts of continuous cross section, such as rods or beams.)
The tool mentioned in the preceding paragraph is typically made of a metal (e.g., aluminum, steel, or Kovar, depending on the part shape) or composite and may require months and many thousands of dollars to make, depending on the complexity and size of the part. (Kovar is a nickel-cobalt alloy with a low thermal expansion coefficient, which makes Kovar useful for tooling for curing composites because a low amount of tool expansion during heating or curing of the composite part limits stress on the part, while a large amount of tool expansion during heating or curing of the composite part can stress the part. Instead of Kovar, any other suitable low-expansion alloy may be used.) The time and cost required to make a tool can be a major impediment in the fabrication of composite parts for prototyping or small production runs, where quick response times can be crucial and the cost of the tool cannot be amortized over many parts.
Electron-beam curing, which is also known as “e-beam curing,” is a relatively new method for curing composite materials that has stimulated significant interest. Curing a part using an e-beam allows curing at low temperatures and much faster cycle times, which reduces processing costs, equipment costs, and tooling costs. Current e-beam curing processes either utilize a high-energy beam (typically 2 to 10 MeV) to cure the entire thickness of the part (that has been laid up over a tool or injected into a tool such as a mold) at one time, or use a lower energy beam (e.g., 300 to 400 keV) to cure one ply of pre-impregnated fiber as it is layed down over a shaped tool. All these approaches require the fabrication of a tool.
The subject invention can eliminate the need to use a tool (such as a mold, die, mandrel, or any other type of tool) to make a continuous fiber-reinforced composite part. A three-dimensional part can be made directly from a computer design file, using the same raw material that is kept on hand for fabricating any composite part, regardless of shape and design. (Of course, the raw materials used would have to be consistent with the requirements of this invention.) This capability could dramatically decrease response time and the cost to fabricate prototypes or small quantities of parts.
Rapid Prototyping
Rapid prototyping (also called “desktop manufacturing” or “free-form fabrication”) has been a very active area in the last 15 years, with a variety of approaches in use or under development. Stereolithography (SLA), as described in U.S. Pat. No. 4,575,330 (which is incorporated herein by this reference), was one of the earliest rapid prototyping methods and utilizes a laser-generated ultraviolet (UV) beam to selectively cure UV-curable acrylate resins. However, the available UV-curable resins have much lower mechanical properties (strength, toughness, glass transition temperature, etc.) than most recently developed electron-beam-curable resins; and since UV radiation does not penetrate the material as deeply, especially when carbon fibers are included, it cannot be used to fabricate carbon-fiber-reinforced composites with thermal and mechanical properties that would be obtainable using the subject invention. Selective Laser Sintering (SLS) was also an early rapid-prototyping method; it operates by using a laser to selectively fuse powder of the working material, adding another layer of powder, and building a part up in this manner. An important advantage of SLS compared to many other techniques is its ability to use a variety of materials, including wax, nylon, polycarbonate, and even (in a limited way) metal. However, SLS does not work well on thermoset plastics (which tend to have superior mechanical and thermal properties compared to thermoplastics), since these materials cannot be melted without significant chemical degradation once they have been cured. Other rapid-prototyping methods include: Three-Dimensional Printing, in which a binder is sprayed in controlled manner onto a bed of particles; Fused Deposition Modeling, in which a thermoplastic filament is melted and applied similarly to squeezing toothpaste from a tube; and other approaches. However, none of these rapid-prototyping approaches is capable of fabricating a thermoset plastic part with thermal and mechanical properties similar to epoxy.
A continuous fiber-reinforced organic-matrix material consists of a resin made of organic material (such as a thermoplastic or thermoset, as opposed to ceramic or metal) which is reinforced by long aligned fibers (e.g., fibers having any suitable size, including, but not necessarily limited to, fibers having a median length of at least about 2 cm, at least about 2.5 cm, at least about 2.6 cm, at least about 3 cm, at least about 4 cm, at least about 5 cm, at least about 10 cm, at least about 13 cm, at least about 15 cm, or at least about 20 cm and optionally having a median length of less than about 50 m, less than about 25 m, less than about 10 m, less than about 5 m, less than about 200 cm, less than about 150 cm, less than about 100 cm, less than about 50 cm, or less than about 25 cm), which optionally extend approximately over the length, width, or depth of the part (these may be straight fibers or fibers woven into a fabric and may have a size approximately equal to the length, width, or depth of the part). By contrast, a discontinuous fiber-reinforced composite is reinforced by small fibers, typically less than about 2 cm long, which are often oriented randomly but may also be aligned. Because the reinforcing fibers are much stronger and stiffer than the organic matrix, and because longer fibers can better carry and distribute mechanical loading, a part reinforced by continuous fibers is much stronger and stiffer than a part reinforced by short fibers. However, fabricating parts reinforced with long, oriented fibers is more difficult, especially when trying to use a rapid-prototyping method.
Our invention is different from current rapid-prototyping methods (with the possible exception of Laminated Object Manufacturing (LOM)) in its ability to combine two capabilities which together produce parts with mechanical and thermal properties that are dramatically superior to those that can be made with existing rapid-prototyping methods:                1) the ability to incorporate continuous (i.e., long, not short) carbon (not glass) fibers as the reinforcement; and        2) the ability to use thermoset (rather than thermoplastic) resins as the matrix.        
As discussed above, other rapid-prototyping methods (with the possible exception of LOM) may have one or the other of the above capabilities, but none can incorporate both the ability to use continuous carbon fibers and the ability to use a thermoset matrix to make a composite part.
Laminated Object Manufacturing (LOM) is described in U.S. Pat. Nos. 5,730,817 and 5,876,550. As currently practiced, LOM uses heat, not an electron beam, to bond together layers of material that have been cut by a computer-controlled laser. The invention described in this application differs from LOM and, as explained below, has several advantages over LOM:                (1) Because each layer is simultaneously formed and bonded to the previous layer in the present invention, the integrity of the bonding between layers will be better in the present process than in LOM; and, thus, the present process will yield a higher quality product with fewer voids and better mechanical properties. This is especially important because there is no flow of the e-beam curable matrix during cure.        (2) Because the present process does not require that layers be formed separately and then picked up and placed onto the workpiece, the present process is more flexible than LOM. For example, the present process allows reinforcement only in selected areas, thereby allowing formation of very thin layers and avoiding indexing issues that can contribute to dimensional inaccuracies.        (3) Because reinforcing fibers for one or more layers can be laid down before introduction and curing of the e-beam curable resin matrix in the present invention (by contrast with LOM), introduction of z-direction reinforcing short fibers can be accomplished; and those fibers can then be immersed in resin and the surrounding resin cured, thereby completely integrating the z-direction fibers into the part.        